Magnetic Ink Character Recognition (MICR) technology is well-known. MICR inks contain a magnetic pigment or a magnetic component in an amount sufficient to generate a magnetic signal strong enough to be readable via MICR. Generally, the ink is used to print all or a portion of a document, such as checks, bonds, security cards, etc. For example, most checks exhibit an identification code area, usually at the bottom of the check. The characters of this identification code are usually MICR encoded. The document may be printed with a combination of MICR-readable ink and non-MICR-readable ink, or with just MICR-readable ink. The document thus printed is then exposed to an appropriate source or field of magnetization, at which time the magnetic particles become aligned as they accept and retain a magnetic signal. The document can then be authenticated by passing it through a reader device, which detects or “reads” the magnetic signal of the MICR imprinted characters, in order to authenticate or validate the document.
There are numerous challenges in developing a MICR inkjet ink. First, most if not all, inkjet printers limit considerably the particle size of any particulate components of the ink, due to the very small size of the inkjet print head nozzle that expels the ink onto the substrate. The size of the inkjet head nozzles are generally on the order of about 40 to 50 microns, but can be less than 10 microns. This small nozzle size dictates that the particulate matter contained in any inkjet ink composition intended for use in an inkjet printer must be of a very small particle size, in order to avoid nozzle clogging problems. However, even when the particle size is smaller than nozzle size, the particles can still agglomerate, or cluster together, to the extent that the size of the agglomerate exceeds the size of the nozzle, resulting in the nozzle being blocked. Additionally, the particulate matter may be deposited in the nozzle during printing, thereby forming a crust that results in nozzle blockage and/or imperfect flow parameters.
Another concern in the formulation of MICR inkjet inks is that the ink must be fluid, and not dry. Thus, an increase in pigment size causes a corresponding increase in density, thereby making it difficult to maintain the pigments in suspension or dispersion within a liquid ink composition.
MICR inks contain a magnetic material that provides the required magnetic properties. It is imperative that the magnetic material retains a sufficient charge so that the printed characters retain their readable characteristic and are easily detected by the detection device or reader. The magnetic charge retained by a magnetic material is known as “remanence.” The “coercive force” of a magnetic material refers to the magnetic field H, which must be applied to a magnetic material in a symmetrical, cyclicly magnetized fashion, to make the magnetic induction B vanish. The coercivity of a magnetic material is thus the coercive force of the material in a hysterisis loop, whose maximum induction approximates the saturation induction. The observed remanent magnetization and the observed coercivity of a magnetic material depend on the magnetic material having some anisotropy to provide a preferred orientation for the magnetic moment in the crystal. Four major anisotropy forces determine the particle coercive force: magnetocrystalline anisotropy, strain anisotropy, exchange anisotropy, and shape anisotropy. The two dominant anisotropies are: 1) shape anisotropy, where the preferred magnetic orientation is along the axis of the magnetic crystal, and 2) magnetocrystalline anisotropy, where the electron spin-orbit coupling aligns the magnetic moment with a preferred crystalline axis.
The magnetic material must exhibit sufficient remanence once exposed to a source of magnetization, in order to generate a MICR-readable signal and have the capability to retain the same over time. Generally, an acceptable level of charge, as set by industry standards, is between 50 and 200 Signal Level Units, with 100 being the nominal value, which is defined from a standard developed by ANSI (the American National Standards Institute). A lesser signal may not be detected by the MICR reading device, and a greater signal may also not give an accurate reading. Because the documents being read employ the MICR printed characters as a means of authenticating or validating the presented documents, it is imperative that the MICR characters or other indicia be accurately read, without skipping or mis-reading any characters. Therefore, for purposes of MICR remanence should be at least a minimum of 20 emu/g. A higher remanence value corresponds to a stronger readable signal.
Remanence tends to increase as a function of particle size and the density of the magnetic pigment coating. Accordingly, when the magnetic particle size decreases, the magnetic particles tend to experience a corresponding reduction in remanence. Achieving sufficient signal strength thus becomes increasingly difficult as the magnetic particle size diminishes and the practical limits on percent content of magnetic particles in the ink composition are reached. A higher remanence value will require less total percent magnetic particles in the ink formula, improve suspension properties, and reduce the likelihood of settling as compared to an ink formula with higher percent magnetic particle content.
Additionally, MICR inkjet inks must exhibit low viscosity, typically on the order of less than about 15 cP or on the order of about 2-8 cP at jetting temperature (whereby the jetting temperature ranges from about 25° C. to about 140° C.), in order to function properly in both drop-on-demand type printing equipment, such as thermal bubble jet printers and piezoelectric printers, and continuous type print mechanisms. The use of low viscosity fluids, however, adds to the concerns of successfully incorporating magnetic particles into an ink dispersion because particle settling will increase in a less viscous, thinner fluid as compared to a more viscous, thicker fluid.
Magnetite (iron oxide, Fe3O4) is a common magnetic material used in MICR inkjet inks. Magnetite has a low magnetocrystalline anisotropy, K1, of −1.1×104 J/m3. An acicular crystal shaped magnetite, in which one crystal dimension is much larger than the other, has an aspect ratio of the major to minor size axis of the single crystal (Dmajor/Dminor) of 2:1 or larger, helps to augment the magnetic remanence and coercivity performance in inks. Acicular magnetite is typically 0.6×0.1 micron in size along the minor and major axis, respectively, and has a large shape anisotropy (6/1). Typical loading of iron oxide in inks is about 20 to 40 weight percent. However, due to the larger sizes and aspect ratio of acicular crystal shaped magnetite particles, they are difficult to disperse and stabilize into inks, especially for use in inkjet printing. Moreover, spherical or cubic magnetites are smaller in size (less than 200 nm in all dimensions), but have low shape anisotropy (Dmajor/Dminor) of about 1. Consequently, because of the low overall anisotropy, spherical or cubic magnetite have lower magnetic remanence and coercivity, and loadings higher than 40 weight percent are often needed to provide magnetic performance. Thus, while spherical and cubic magnetite have the desired smaller particle size of less than 200 nm in all dimensions, the much higher loading requirement also makes them very difficult to disperse and maintain a stable dispersion. Moreover, such high loadings of the inert, non-melting magnetic material interfere with other ink properties, such as adhesion to the substrate and scratch resistance. Consequently, this worsens the suitability of magnetites for inkjet printing inks.
Additionally, because magnetite has a specific gravity of approximately 7, magnetite has a natural tendency to settle to the bottom of a fluid ink composition. This results in a non-homogenous fluid having an iron oxide-rich lower layer and an iron oxide-deficient upper layer. Moreover, suitable inkjet oxides must generally be hydrophilic in nature in order to provide good dispersion characteristics, and to provide good emulsion properties. The latter parameters relate directly to the ability of the magnetic particle to exhibit minimum settling and to further demonstrate the proper wetting of the magnetic particle with the other water-soluble ingredients generally present in an inkjet ink composition.
The problems commonly associated with using iron oxide in MICR inkjet inks have been addressed in several different ways. For example, using a combination of surfactants in conjunction with a very small particle size metal oxide component, aimed at maintaining a useful suspension or dispersion of the magnetic component within the ink composition, is known. Another means of achieving an inkjet ink suitable for use in inkjet printers, and also for generating MICR-readable print, is to coat the metal magnetic material with a specific hydrophilic coating to help retain the particulate magnetic metal in suspension.
Still yet, another type of ink used for MICR inkjet printing, is xFerrone™ (iron complex pigment) inks, which are aqueous inks commercialized by G7 Productivity Systems, Inc. (VersaInk™). These inks are compatible with HP®, Canon®, Lexmark®, Dell® and Epson® printers, and have a variety of uses, such as, for example, ensuring reliable scanning of checks, and eliminating delays at a store checkout line. However, these inks do not exhibit the properties of including a reduce sized magnetic material particle that has excellent magnetic pigment dispersion and dispersion stability, while maintaining excellent magnetic properties, and a reduced particle loading requirement. This is because the major/minor axis of the magnetic particles used in such conventional inks must have at least a 2:1 ratio, and therefore, the particle size of the acicular magnetite is 0.6 micron for the major axis. This results in poor dispersion and poor dispersion stability.